These days, mobile telecommunications businesses such as automotive telephones and mobile telephones are developing dramatically. In many countries in the world, various mobile telecommunications systems are already in service.
Access methods for mobile telephones currently include TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access).
The TDMA standards include PDC (Personal Digital Cellular) used in Japan, EGSM (Extended Global System for Mobile Communications) and GSM1800 (Global System for Mobile Communications 1800) mainly used in Europe, and DAMPS (Digital Advanced Mobile Phone Service) mainly used in the United States of America.
Meanwhile, the typical CDMA standards are IS-95 (Interim Standard-95) and W-CDMA (Wide-band CDMA).
In these mobile telecommunications systems, signal processing parts of many mobile terminals use semiconductor field-effective transistors (FETs). Particularly for mobile terminals for which the portability is the most important, monolithic microwave ICs (MMICs) using GaAsFETs are under vigorous development. An MMIC is a semiconductor integrated circuit element which can realize a compact size, low-voltage driving and low consumption energy at the same time.
Among such semiconductor integrated circuits, development of high-frequency switch circuits which switch a path for a high-frequency signal particularly within a mobile terminal is becoming important.
Specific examples of switching of a path for a high-frequency signal include switching of a path for supplying a high-frequency signal received at an antenna to a receiver part and switching of a path for outputting a high-frequency signal which has been outputted from a sender part to an antenna.
Further, specific examples of switching of a path for a high-frequency signal within a multi-mode mobile telephone which comprises a plurality of sender parts and a plurality of receiver parts include, in addition to the switching described above, switching of a path for connecting one of the plurality of sender parts with an antenna and switching of a path for connecting one of the plurality of receiver parts with the antenna. In addition, since a mobile telephone comprises a plurality of antennas, that is, an internal antenna and an external antenna, switching of a signal path also include switching of a path to selectively use the plurality of antennas.
By the way, in order to use an FET as a switching device for switching of a signal path, it is necessary to control a bias voltage which is applied upon a gate terminal of the FET. For example, when a gate bias which is sufficiently higher than a pinch-off voltage is applied upon the gate terminal, a low impedance develops between the drain and the source and the FET is controlled to the ON-state. On the contrary, when a gate bias which is sufficiently lower than the pinch-off voltage is applied upon the gate terminal, a high impedance develops between the drain and the source and the FET is controlled to the OFF-state.
In this manner, a voltage at a gate terminal of a GaAsFET is changed, thereby switching a signal path and realizing the function of an antenna switch.
One example of a switch circuit having such a structure is an SPDT (Single Pole Dual Throw) switch which is formed by combining one series FET and one shunt FET for a signal path. This structure allows the shunt FET which is in the ON-state to draw an RF (Radio Frequency) signal leaking through a capacitance component from the series FET which is in the OFF-state to the ground, and hence, provides high isolation.
Meanwhile, a digital mobile telephone complying with the TDMA telecommunication method uses a DPDT (Dual Pole Dual Throw) switch for switching between an accessory antenna and an external antenna and for switching between a sender part and a receiver part which are disposed within the mobile telephone.
A DPDT switch is formed using an FET, and comprises a first and a second input terminals and a first and a second output terminals. The DPDT switch switches between outputting of a signal from the first and the second input terminals respectively to the first and the second output terminals and outputting of a signal from the first and the second input terminals respectively to the second and the first output terminals.
In these switches, for the purpose of operating the FET, a voltage equal to or higher than the pinch-off voltage is applied upon the FET and a low impedance develops, while a voltage equal to or lower than the pinch-off voltage is applied upon the FET and a high impedance develops, as described earlier.
The maximum sending power (Pmax) which can be handled when an FET is used as a switching element is expressed by the following equation:Pmax=2|Vc−Vp|2/Z
Vc: the control voltage at the gate terminal
Vp: the pinch-off voltage
Z: the load impedance
Hence, in order to increase the maximum sending power, the difference between the control voltage at the gate terminal (Vc) and the pinch-off voltage (Vp) may be increased.
However, as for voltages of batteries for mobile telephones, there is a limit against an increase of the difference between the gate terminal control voltage (Vc) and the pinch-off voltage (Vp), and therefore, it is not possible to obtain sufficient maximum sending power.
It is a voltage raising circuit that is necessary to increase the gate terminal control voltage (Vc). If a voltage of a battery of a mobile telephone is used as it directly is, the gate terminal control voltage (Vc) will be restricted up to the battery voltage. However, use of a voltage raising circuit makes it possible to increase the voltage of the gate terminal control voltage (Vc) beyond the battery voltage. In this manner, it is possible to ensure a large difference between the gate terminal control voltage (Vc) and the pinch-off voltage (Vp). From these, use of a voltage raising circuit is important in order to eliminate the restrictions related to the maximum sending power.
An antenna switch semiconductor integrated circuit using a voltage raising circuit is one described in Japanese Patent Application Laid-Open Gazette No. H11-55156, for instance. In this antenna switch semiconductor integrated circuit, there are a decoder circuit, a drive circuit, an oscillation circuit and a double voltage generator circuit disposed on an MOS integrated circuit semiconductor chip. As the double voltage generator circuit is used, a voltage supplied from outside can be changed internally to a desired high voltage.
Use of the double voltage generator circuit thus makes it possible to internally obtain a high voltage, and hence, increase the gate terminal control voltage (Vc). Hence, the maximum sending power (Pmax) can be enhanced. This allows fabrication of an antenna switch semiconductor integrated circuit which is capable of handling a high output.
Other example of a conventional antenna switch semiconductor integrated circuit may have a structure shown in FIG. 13 for example. Operations of a conventional antenna switch semiconductor integrated circuit will now be described with reference to FIG. 13.
First, a GaAs semiconductor chip 2 and an Si semiconductor chip 3 are mounted on an IC package, and these operate as an antenna switch semiconductor integrated circuit.
Switches 4 through 7 are formed by GaAsFETs and fabricated on the GaAs semiconductor chip 2. These switches 4 through 7 are connected with external signal terminals 24 through 27 and an antenna 28 via external capacitors 19 through 23.
Further, switches 8 through 11 are formed by GaAsFETs and fabricated on the GaAs semiconductor chip 2. One ends of these switches 8 through 11 are connected between the switches 4 through 7 and the external capacitors 19 through 22, and the other ends of the switches 8 through 11 are grounded via capacitances 12 through 15.
In this structure, when a signal path from the external signal terminal 24 to the antenna 28 is to conduct for instance, the switches 4 and 9 through 11 are turned on and the remaining switches 5 through 8 are turned off. This causes the signal path from the external signal terminal 24 to the antenna 28 to conduct and provides sufficient isolation from the remaining external signal terminals 25 through 27.
Control of these switches 4 through 11 is realized by a logic circuit 17 which is formed on the Si semiconductor chip 3. To be specific, in accordance with the states of a plurality of control input signals INA and INB fed to control terminals 29 and 30, the logic circuit 17 selectively makes the switches 4 through 11 conduct.
An ordinary antenna switch semiconductor integrated circuit can be controlled using these structures. However, for the purpose of controlling a high output signal, the maximum sending power (Pmax) needs be increased as described above. To this end, it is necessary to increase the gate terminal control voltage (Vc) at the switches 4 through 11. Hence, a voltage raising circuit 16 is necessary.
The voltage raising circuit 16 performs a voltage raising operation based on an oscillation signal which is supplied from an oscillation circuit 18C. The voltage raising circuit 16 is formed on the Si semiconductor chip 3, together with the oscillation circuit 18C and the logic circuit 17. Although a specific structure of the oscillation circuit 18C will not be described here, the oscillation circuit 18C is realized by adding a power source voltage to a terminal 108 at all times in the structure which is shown in FIG. 2 for example.
Based on the oscillation signal from the oscillation circuit 18C described above, the voltage raising circuit 16 internally raises an external voltage supplied through a power source voltage terminal 31, and then supplies thus raised voltage to the logic circuit 17 as an operation power source voltage. In this fashion, it is possible to obtain a high output voltage from the logic circuit 17. Using this voltage as the gate terminal control voltage (Vc), a high output signal can be controlled.
The structure according to the conventional example above uses a voltage raising circuit in order to obtain a high voltage. However, where a voltage raising circuit is used to obtain a high voltage, the voltage raising circuit is always operating instead of operating only at the time of sending which demands a high voltage. Because of this, a current needed to operate the voltage raising circuit is always flowing in the antenna switch semiconductor integrated circuit. Hence, if the antenna switch semiconductor integrated circuit is incorporated within a mobile telephone, the mobile telephone will consume more current than needed and a standby time and a call time of the mobile telephone will be shortened.